Electromagnetic flowmeter

ABSTRACT

A noninvasive electromagnetic flowmeter particularly adapted to measure arterial blood flow in human beings is provided. A homogenous magnetic field is produced in the region of the artery under measurement by means of at least one large coil remote from the skin. Blood flow induced signals are sensed by electrodes placed on the skin adjacent the artery and fed to a measurement and control circuit. Electrocardiogram signals sensed by a second pair of electrodes placed on the body near the heart synchronize the operation of this system. A preferred embodiment of control and measurement system comprises a pair of auxiliary electrodes located where a strong and sharp cardiogram pulse can be repeatedly obtained to be used as a synchronizing signal and as a clock, at least one coil of large enough size having enough turns of wire to be adequate for the production of a strong and homogeneous magnetic field in the region of the artery under study, a source of DC current to feed that coil, means to reverse that DC current after a given number of heart cycles in accordance with a program controlled by the synchronizing signal, a pair of measuring electrodes placed on the skin adjacent the artery, an amplifier to amplify the signal appearing between said measuring electrodes, with filtering means to eliminate the DC unbalance of the electrodes and the high frequency noise, means to paralyze the amplifier and to reverse its output during each current reversal, and means synchronized by the synchronizing signal to average the pulsatile signal measured between the measuring electrodes during successive heart cycles so as to extract the repetitive wave shape of the blood flow pulses from the large random noise in which it is otherwise buried.

[451 May 2,1972

[73] Assignee:

[54] ELECTROMAGNETIC FLOWMETER [72] Inventors: Henri Georges Doll, Newyork; Hans J.

Broner, Glendale, both of N.Y.

Doll Research Incorporated, New York, N.Y.

[22] Filed: Aug. 24, 1970 [21] Appl.No.: 66,240

[52] US. Cl. ..128/2.05 F, 73/194 EM [5]] Int. Cl. ..A6lb 5/02 [58]Field of Search ....l28/2.05 F, 2.05 R, 2.05 V,

128/205 P, 2 R; 73/194 EM Spencer. M. P. et al., 1. R. E. Trans. onMedical Electronics, Dec, 1959. Pp- 220-227.

Primary ExaminerRichard A. Gaudet Assistant ExaminerKyle L. HowellAnomey1(enyon & Kenyon Reilly Carr & Chapin [57] ABSTRACT A noninvasiveelectromagnetic flowmeter particularly adapted to measure arterial bloodflow in human beings is provided. A homogenous magnetic field isproduced in the region of the artery under measurement by means of atleast one large coil remote from the skin. Blood flow induced signalsare sensed by electrodes placed on the skin adjacent the artery and fedto a measurement and control circuit. Electrocardiogram signals sensedby a second pair of electrodes placed on the body near the heartsynchronize the operation of this system.

A preferred embodiment of control and measurement system comprises apair of auxiliary electrodes located where a strong and sharp cardiogrampulse can be repeatedly obtained to be used as a synchronizing signaland as a clock, at least one coil of large enough size having enoughturns of wire to be adequate for the production of a strong andhomogeneous magnetic field in the region of the artery under study, asource of DC current to feed that coil, means to reverse that DC currentafter a given number of heart cycles in accordance with a programcontrolled by the synchronizing signal, a pair of measuring electrodesplaced on the skin adjacent the artery, an amplifier to amplify thesignal appearing between said measuring electrodes, with filtering meansto eliminate the DC unbalance of the electrodes and the high frequencynoise, means to paralyze the amplifier and to reverse its output duringeach current reversal, and means synchronized by the synchronizingsignal to average the pulsatile signal measured between the measuringelectrodes during successive heart cycles so as to extract therepetitive wave shape of the blood flow pulses from the large randomnoise in which it is otherwise buried.

15 Claims, 2 1 Drawing Figures MEASUREMENT Patented May 2, 19723,659,591

5 Sheets-Sheet l MEASUREMENT 411 am M01. srsrewi 40 FIGI MEASUREMENT andL'O/VTPUL 5 y 5 7' EM FROM LOG/C 94 cup ma 94 MP CIRCUIT OUTPUT I00 1DPDT FIG.4

INVENTORS HENRI GEORGES DOLL HANS J1 BRONER A TTORWEYS ELECTROMAGNETICFLOWMETER FIELD OF THE INVENTION This invention relates toelectromagnetic flowmeters and more particularly to a transcutaneouselectromagnetic flowmeter for monitoring the flow pulse in the bloodvessels of living beings.

BACKGROUND OF THE INVENTION In general, electromagnetic flowmeters forthe measurement of the blood flow have been invasive, i.e., they haverequired surgical exposure of the blood vessel under measurement andimplantment of at least part of the sensing device about such vessel.Such meters are severely limited in their application and not suitablefor clinical use due to the necessity of such surgical procedures andthe attendant sterility problems. In addition, not all blood vessels maybe exposed in this way since those that have become atherosclerotic arebrittle and may be damaged in the implanting procedure. The ugly scarscaused by the surgical procedure also dictate against the use ofinvasive meters except in the most critical cases. For these reasons theinvasive flowmeter are rarely used on human beings and are mostly usedfor experiments on anesthetized animals.

Besides these physical limitations in known electromagnetic flow meters,such meters have also had severe drawbacks in the electronic sensing andmeasurement system. Since the desired blood flow signal is mixed in withunwanted noise signals created in the body and in the electronic systemitself, it has been found difficult to eliminate the noise in order toobtain an accurate blood flow reading. Such noise includes extraneoussignals caused by poor placement of the flowmeter around the vesselunder measurement, noise created by electrical interaction of theelements of the flowmeter sensor and between the sensor and the tissuewith which it is in contact, quadrature effect in sine wave typeflowrneters and transformer spikes in square wave type flowmeters.

It is desirable that the blood flow measurement by the flowmeter betaken during a period of constant magnetic field, and 7 that theflowmeter be simple to operate and be capable of monitoring the bloodflow in a number of vessels in the same individual.

OBJECTS OF THE INVENTION It is thus an object of the present inventionto provide a noninvasive electromagnetic flowmeter which measures theblood flow in the vessels in living beings without surgical implantationof sensors in the being.

It is a further object of the present invention to provide anon-invasive electromagnetic flowmeter which provides an averagedmeasurement of the blood flow pulse over a number of heart cycles.

It is another object of the present invention to provide a non-invasiveelectromagnetic flowmeter which efiectively cancels out noise created bythe electrocardiogram signal between the blood flow measuring electrodesover a number of heart cycles and which eliminates random noise duringthat number of cycles by averaging the pulses.

It is yet another object of the present invention to provide anon-invasive electromagnetic flowmeter wherein the measurement andcontrol system is synchronized by means of a reference electrocardiogramsignal.

It is still yet another object of the present invention to provide anelectromagnetic flowmeter wherein flow induced signals are only measuredwhile the magnetic field is constant and wherein the measurement systemis paralyzed reversal of the magnetic field.

It is still a further object of the present invention to provide anon-invasive electromagnetic flowmeter which is simple to operate andwhich may be used to monitor the blood flow in a plurality of vesselssimultaneously or sequentially.

SUMMARY OF THE INVENTION According to a first aspect of the presentinvention, a homogeneous magnetic field is created in the region of theblood vessel under measurement by magnetic field producing meanssituated external of the living being. Blood flow induced electricalsignals are sensed by means of sensors placed on the skin of the beingin the vicinity of the vessel.

According to one other aspect of the invention these signals are fed toa measurement and control system whose operation is synchronized bymeans of electrocardiogram signals sensed by a second set of sensorsplaced on the skin of the being in a location where theelectrocardiogram signal is strong and sharp, as, for example, in thevicinity of the heart.

In a preferred embodiment of the measurement and control system, anelectromagnetic coil provides a substantially homogeneous magnetic fieldat the artery to be studied, the

coil being fed a stable DC current during blood flow measurement. Aplurality of flow induced signals are measured by the first set ofsensors and are averaged in an analog measuring circuit to produce anaveraged pulse which is a function of the rate of flow in the arteryunder measurement. The electrocardiogram signals detected by the secondset of sensors are utilized to reverse the field of the electromagnetthrough a suitable control circuit, to reverse the polarity of theanalog measuring circuit and to paralyze the measuring circuit duringtheir reversal.

This reversal is accomplished after a given number of elec trocardiogrampulses in order that an equal number of cardiogram signals of oppositepolarity be stored in the analog circuit thus causing cancellation ofthe electrocardiogram component of the flow induced signal in the analogmeasuring circuit. Due to the random nature of the other noise it iseliminated by being averaged out in the analog measurement circuit. I

According to a further aspect of the invention the analog measuringcircuit is paralyzed during field reversal.

DESCRIPTION OF THE DRAWINGS FIGS. 1 and la are diagrammatic views of apreferred embodiment of electromagnetic flowmeter according to thepresent invention illustrating measurement of bloodflow respectively inthe carotid and femoral arteries of a human be- FIG. 2 is a blockdiagram of a preferred embodiment of measurement and control electricalsystem of the flowmeter of FIG. 1.

FIGS. 3a and 3b are illustrative block diagrams of the control logiccircuitry of FIG. 2.

FIG. 4 is an illustrative schematic diagram of the trapezoid functiongenerator of FIG. 2.

FIG. 5 is an illustrative schematic diagram of the current source ofFIG. 2.

FIG. 6 is a waveform diagram of one of the signals generated in thecircuit of FIG. 4.

FIG. 7 is a waveform diagram of several of the signals generated in thecontrol logic circuit of FIG. 30.

FIG. 8 is a representative waveform diagram of the PQRSTelectrocardiogram signal of the human heart.

FIG. 9 is a waveform diagram of various signals generated in thecircuitry of FIG. 2.

FIG. 10a is a waveform diagram of an actual electrocardiogram signal ofa human heart measured between two electrodes across the femoral artery.

FIG. 10b is an example of the residual noise signal after reversing andaveraging a wave of actual electrocardiogram signals and other noisesover a period of approximately ten minutes in the flow meter of FIG. 1but without applying the magnetic field, and therefore without bloodflow signals.

FIG. l0c-h are waveform diagrams of averaged blood flow pulses asmeasured in the femoral artery of a human being.

FIG. 11 is sectional elevational view of a skin electrode that may beused-according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS Referring now to thedrawings'there is shown in FIG. 1, a general diagrammatic view of apreferred embodiment of blood flowmeter according to the presentinvention used to measure the blood flow rate through the carotid arteryof a patient. As shown, a patient is lying in a prone position on anexamination table 12. Since it is desired to measure the blood flow ratethrough the right carotid artery 14 of the patient, a

homogeneous magnetic field is created in the region of artery 14 bymeans of electromagnetic 18. The strength of this magnetic field must besufficiently large so that a detectable electrical signal induced by thepassage of blood in the carotid artery through the magnetic field ispresent at the skin surface of the neck;

' Electromagnet 18 is thus comprised of a coil of heavy gauge wireadapted to carry large amounts of current to create a high intensitymagnetic field. It has been found that for the field to be homogeneousin the region of the artery under measurement, the diameter of the coilshould be at least twice the distance of the coil from the artery. Ahomogeneous magnetic field is desirable in order to avoid false readingsthat may occur if the patient moves his head and thereby changes thelocation or orientation of the artery in the field. It is also desirablethat the magnetic field be roughly perpendicular to the skin and theartery. v In order that measurement may be made of blood flow indifi'erent arteries of the patient it may be desirable that more thanone electromagnet such as 18 be provided, as for example one for thecarotid arteries region and another one for the femoral arteries region.g

- Current is supplied to electromagnet 18 through electrical conductors20 from measurement and control system 24. A pair of electrodes 26 isplaced on the neck of the patient 10 on either side of the right carotidartery. Electrodes 26 sense an electrical signal that has been inducedby the flow of blood in the carotid artery through the homogeneousmagnetic field present in the region of the artery. This flow inducedsignal is fed to system 24 by means of shielded twisted wire pair 28connected between electrodes 26 and terminals 30 of system V A secondpair of electrodes 26a is placed on the chest of patient 10- and sensethe large electrocardiogram signals produced by the heart at thatlocation. Shielded twisted wire pair 32 connected between chestelectrodes 26a and terminals 34 of system 24 carries these signals tosystem 24.

Since it is desirable that system 24 be at the same ground 7 potentialas patient 10, ground electrode 26b is placed onthe leg or arm ofpatient 10 and connected to ground terminal 36 of system,24 by conductor38. Electrode 26b is preferably placed on a portion of the body which isremoved from electrodes 26 and 26a.

The'blood flow diagram producedby system 24 may be recorded on a stripchart recorder 40, may be displayed on a visual display device such asan oscilloscope (not shown) or may be recorded on a suitable recordingmedia such as magnetic tape either directly or after being converted todigital data for use in a digital computer by well known analog-digitalconversion devices. I

5 FIG. la shows the relative positioning of electromagnet 18 for themeasurement of blood flow throughthe right femoral artery 42 of patient10 and also the relative positioning of electrodes 26 for sensing theflow induced signal.

In general, the blood flow induced signal sensed by electrodes 26 .willcontain a number of noise components in addition to the component whichis a function of the blood flow in the artery. The main noise componentis the electrocardiogram signal produced by the heart which can have astrength many times that of the flow induced signal especially at anartery close to the heart such as the carotidartery. It has been foundthat, since it is not necessary to display each individual blood flowpulse, these pulses with proper synchronizing based on the referencecardiogram may be averaged over a long period, such as one or severalminutes to progressively build up an average pulse with the wave formand cleared from noise. The cardiogram noise cancels out Referring nowto' FIG. 2 there is shown a preferred embodiment of measurement andcontrol circuit 24. The blood flow induced signal sensed by electrodes26 is fed by wire pair 28 to the input terminals 30 cf preamplifier 44.As described above, a common ground is maintained between the patientand circuit 24 by means of electrode 26b placed on the skin of thepatient, ground wire 38 and ground terminal 36 of preamplifier 44. Afterthe flow induced signal has been amplified by preamplifier 44 it isfurther amplified by DC amplifier 46. This signal is then filtered bylow pass filter 48 which filters out all high frequency noise componentsincluding any noise component that may be produced by the power linefrequency.

The electrocardiogram signal sensed by electrodes 26a is fed toterminals 34 of preamplifier 50 by twisted wire pair 32. After thecardiogram signal has been amplified by preamplifier 50 it is fed toaverager 52 and thence to control logic circuit 54 to be used tosynchronize the operation of the various functions of system 24.

A reversible steady state DC current is supplied to field coil 56 ofelectromagnet 18 by means of trapezoid function or:

ramp generator 58 connected to current source 60 which in turn suppliescurrent to coil 56. Generator $8 is connected to' control logic circuit54 which controls the operation thereof.

After the blood flow signal is filtered by filter. 48, his fed through adead time relay 62 and a double pole double throw DH-9 Waveform Eductormanufactured by Princeton Ap-,

plied Research Corporation of Princeton, New Jersey, a

' repetitive input waveform to be measured is divided into increments of.equal duration and stored in a 100 channel capacitor memory. After asufficient number of repetitions the voltage level on each capacitorwill be proportional to the average'value for that segment of thewaveform. Random noise and. non-synchronous signals arecliminated andthe desired signal can be read out on a chart recorder or oscilloscope.1

The operation of the electromagnetic flow meter as shown in FIGS. 1 and2 may be understood by referring to the waveform diagrams of FIG. 9.FIG. 9a shows aseries of reference electrocardiogram signals having thegeneral shape of the waveform PQRST shown in FIG. 8. Theelectrocardiogram signal is characterized by a sharp spike at R which isused to trigger measurement and control system 24 .and synchronize theoperation of the various components thereof.

This synchronization on the R line is desirable-since the heart bereversed by activating relay 64. Control logic circuit 54 I also causesrelay 62 to stay open from R to R, so that no signals are fed toaverager 52 during this period of time. This is to prevent unwantednoise caused by the field reversal to be fed to averager 52 and to allowthe system to return to a steady state condition before measurement ofthe blood flow signals is resumed. As shown in FIG. 9d the current isreversed at R and the system allowed to stabilize between R and R At Rcontrol logic circuit 54 causes relay 62 to close (FIG. 9d) thusallowing the blood flow signals to be fed into averager 52. FIGS. 90 and92 show the two main components of the signal, 9c showing theelectrocardiogram noise component which is to be cancelled out and FIG.92 the desired blood flow induced component which is to be measured.

During the interval from R to R,,, a steady state DC current is suppliedto coil 56 of electromagnet 18 which in turn creates a stable anduniform homogeneous magnetic field in the region of the artery (orarteries) under measurement. Since the averager 52 is operated insynchronism with the flow induced signals by means of the referenceelectrocardiogram signals sensed by electrodes 26a, six flow inducedpulses will be introduced into averager 52 during this interval.

As shown, the direction of the magnetic field and polarity of relay 64cause six pulses having six negative cardiogram components (FIG. 9a andsix positive blood flow components to be loaded on the condensers ofaverager 52.

At R control logic circuit 54 causes generator 58 to reverse, thuscausing the current through coil 56 to reverse and the magnetic field toreverse. Reversal of the polarity of the magnetic field causes areversal of the polarity of the magnetic field induced components, butdoes not affect the polarity of the electrocardiogram components.Circuit 54 also causes relay 64 to operate in order to reverse thepolarity of the signals fed to averager 52 and causes relay 62 to beopened between R and R to prevent the feeding of blood flow signals toaverager 52. The relay operating times are chosen so that the dead-timerelay operates before the others.

By R,, (FIG. 9d the field current has reversed and it is completelystabilized at R At R control logic circuit 54 causes relay 62 to close,to permit blood flow pulses to be fed to averager 52 once again. Duringthe interval from R to R six signals are fed to averager 54. However,since the magnetic field has been reversed as well as the polarity ofthe signals fed to averager 52, the six signals will have six positiveelectrocardiogram components and six positive blood flow components.

This is because the positive blood flow components were reversed inpolarity a total of two times; first, due to a reversal in the polarityof the magnetic field, and second, due to the electrical switching ofthe polarity of the electrical signal fed to the averager 52. Thepositive electrocardiogram components were reversed in polarity a totalof only one time, this reversal being produced by the electricalswitching of the polarity of the signals fed to averager 52. These sixpositive cardiogram signals will cancel out the six negative cardiogramsignals fed in during the interval from R, to R thus eliminating thecardiogram noise component from the desired flow pulse. The six bloodflow components will be added to the previously obtained six flowcomponents to produce 12 blood flow pulses loaded on the condensers ofaverager 52 free of electrocardiogram noise.

At R control logic circuit 54 causes a repetition of the events whichtook place at R Thus, during a cycle of 16 heart beats, 12 pulses haveaccumulated in averager 52 and the signals during four of the 16heartbeats have been ignored. Measurement has only taken place when themagnetic field is constant and has been ignored during periods ofcurrent reversal when conditions are unstable. In this manner, anaccurate measurement of the blood flow pulse is obtained and reliabilityof the measuring circuit is assured.

This cycle is repeated over a long enough period of time to give a goodaverage reading of the blood flow pulse in theartery under measurement.Since the average heart beats in the range of from 60 to 100 beats perminute (under v2 hz.) a read ing over a period of 1-10 minutes shouldprovide an averaged blood flow waveform which is a function of theactual blood flow in the artery under measurement.

As an example, FIG. 10 in which the signal scale is 0.1 microvolt perdivision for all curves, shows the results of a number of actualmeasurements of blood flow in the right femoral artery of a man. FIG.10a is the waveform of the averaged electrocardiograms recorded betweenthe electrodes across the femoral artery in the absence of magneticfield. FIG. 10b is the residual cardiogram noise signal after reversingand averaging equal numbers of positive and negative cardiograrn pulsesand other noise over a period of approximately ten minutes also in theabsence of magnetic field.

FIG. 10c e are resultant blood flow waveforms of three differentmeasurement periods of approximately v10 -minute durations. FIG. 10f andg are resultant waveforms of 560 averaged pulses and FIG. 10h is theresultant waveform of 280 averaged pulses. It will be noted that the sixwaveforms are substantially similar and thus may be said to provide agood indication of the blood flow pulse in the artery being measured.

When measurements are taken at periodic intervals, a decrease in thepeak to peak amplitude between the positive and negative peaks wouldindicate that the blood flow in the artery was decreasing thus giving animportant warning to the physician.

Where blocking condensers are used in the preamplifiers and amplifierswhich amplify the blood flow signal, control logic circuit 54 will causepreamplifier 44 to be paralyzed during current reversal to preventcharging of the condensers by the transients produced during reversaland the residual discharge thereof during the measuring intervals.

Referring now to FIG. 3a there is shown in greater detail one circuitwhich may be used for control logic circuit 54 to control the operationof system 24 over the 16 heart beat cycle described hereinabove and toFIG. 7 which shows waveforms of various signals produced in the circuitof FIG. 3a. As shown in FIG 3a, four flipflop 66a, 66b, 66c and 66d areconnected in sequence and produce the voltage signals shown in FIG. 7.

FIG. 7a shows the voltage X fed to the input of flipflop 660, thisvoltage comprising a series of square pulses which are of the samefrequency as and which have been formed from the R spikes of theelectrocardiogram signals sensed by electrode 260.

As is well known, the voltage signal produced by each successiveflipflop in a chain will have a frequency one-half the frequencyv of thetrigger voltage signal applied to the input of the flipflop. Thus, thevoltage X (FIG. 7b) produced by flipflop 66a has one half the frequencyof triggering voltage X the voltage X (FIG. 70) produced by flipflop 66bhas one half the frequency of triggering voltage X,; the voltage X (FIG.7d) produced by flipflop 660 has one half the frequency produced bytriggering voltage X and the voltage X (FIG. 7e) produced by flipflop66d is one-half the frequency of triggering voltage X Voltage signal Xis supplied to relay amplifier/driver 68 which triggers DPDT relay 64and to driver 70 which triggers generator 58.

Voltage signal X (FIG. 7b) is derived from flipflop 66b and 660 and issupplied to relay driver 72, which drives dead time relay 62.

FIG. 7g shows the current waveform supplied to coil 56 by current source60.

FIG. 3b shows in detail another circuit which may be used for controllogic circuit 54 in order to produce a measuring cycle extending over 64heart beats wherein during each half cycle 4 beats are used to allowstabilization of current during reversal and 28 beats are used tomeasure the blood flow signal during current steady-state conditions.Thus during each measuring cycle 28 negative and 28 positiveelectrocardiogram components are cancelled out in averager 52 and 56positive blood flow pulses are loaded on the condensers of averager 52.

trapezoid function or ramp generator 58. A source of DC such as battery90 is connected to DPDT switch 92 which is connected to operationalamplifier 94 by means of adjustable resistor 96 which is connected tothe negative input terminal of amplifier 94 and through ground to thepositive input terminal thereof. A capacitor 98 is connected between thenegative input and output terminals of amplifier 94. Clipping circuit100 is connected to the output of amplifier 94.

In operation, when a trigger signal is received from logic circuit 54 toreverse DPDT switch 92 the voltage applied to amplifier 94 from battery90 is also reversed. If it is assumed that the gain of amplifier 94 ishigh, its input impedance is high and its output impedance is low, thenthe shape of the output voltage will be determined by the values ofresistor 96 and capacitor 98 according to the following formula,

ovut RC o in where R is the value of resistor 96 C is the value ofcapacitor 98 r E is the value of the voltage of battery 90 E, will havea slope determined by the value of 1/RC. Thus by changing the value ofvariable resistor 96 the slope of E may also be changed Clipping circuit100 determines the voltage at which the ramp isstopped and representsthe steady state voltage which is supplied to current source 60.

FIG. shows schematically a circuit that may be used for current source60. The function of current source 60 is to supplya constant DC currentto coil 56 in order to produce a constant magnetic field duringtheperiod when blood flow is being measured. It is also desirable thatthe current through coil 56 be stabilized before the measurementinterval begins.

Current source 60 basically comprises operational amplifier 102 incascade with power amplifier 104 the output of which is connected tocoil 56..Suitable input resistors 106, 108 and 110 and feedbackresistors 112 and 114 wellknown to those skilled in the art are alsoprovided and will not be described in detail here. As shown the outputof ramp generator 58 is con nected to the positive input terminal ofamplifier 102 by means of resistor 106.

ln'order to speed up the time in which the current through coil 56reaches a steady state value resistor 116 is provided in 3 the outputcircuit of amplifier 104. Since the current passing through coil 56 alsopasses through resistor 116, a negative voltage is developed acrossresistor ll6'which is fed back to the input to effectively decrease thenatural time constant of coil 56. Thus the current on reversal reachesits steady state value quicker than if resistor 116 were not provided.FIG. 6 illustrates the waveforms of the driving voltage and current onreversal.

FlG.'ll shows an electrode which may be used as the sensor electrode 26according to the present invention. The purpose of the electrode is toobtain the maximum electrical contact with the skin on which it isplaced. Electrode 118 comprises cup shaped plastic housing 120 havingmetallic electrode film 122 on the interior thereof covered withconductive jelly 124.

Electrode film 122 is of a non-polarisable nature such as asilver-silver chloride lattice. Lead wire 126 is soldered to electrodefilm 1 22 and carried out through housing 120. The elec-.

trode may besecured to the skin of a patient-by a suitable adhesive 128which may be coated on the bottom of housing 120. a r

The conductive jelly 124 will mold itself to imperfections in r the skinand provide maximum contact between the skin and electrode film 122.

FIG. 12 diagrammatically illustrates the optimum placement of electrodes26 with respect to the artery under measurement. Ideally, the magneticfield B is perpendicular to the 7 skin and homogeneous in. the region ofthe artery. The ideal position P at which one of the electrodes shouldbe placed is that point above the artery where the induced electricalvoltage is at a maximum or minimum. This point may be derived B is themagnetic field strength in Webers per square meter V, is the voltage atpoint P in volts Q is the blood flow in the artery in cubic meters persecond G1 is the electrical conductivity of blood G2 is the electricalconductivity of tissues V, is the slip velocity at the artery wall inmeters per second, which is known to be very close to zero because ofthe laminar flow conditions A is the radius of the artery in meters S isthe distance from the center line of the artery to the ski surface inmeters X is the distance on the skin from point P to a line passingthrough the center of the artery in meters The formula shows thatmaximum and minimum voltages occur at two points on either side of theartery where X S.

Thus, optimally, electrodes 26 should be placed at these points in orderto achieve the greatest blood flow signal.

Although it is desirable that a patient remain in one position whileblood flow is being measured, this may not always be possible and thereis a likelihood especially in measurement cf the blood flow in thecarotid arteries that the patient will move his head during themeasurement cycle. This movement will change the orientation of theblood vessel in the magnetic field thus changing the strength of thesignal-pickedup by the electrodes.

FIG. 13 shows in simplified form a circuit which may be used to controlthe gain of the measurement system in response to moderate positionchanges of a patient. A pickup" coil 130 is placed on the skin betweenelectrodes 26.

In order to assure that the blood flow signal provided to averager 132is maintained at a constant level despite changes of position of theartery and skin in the magnetic field, a voltage divider networkcomprising photoconductive cell 134 and resistor 136 is connected to theinput of the averager. Cell 134 is enclosed within casing 138 with abulb.l40. Coil 130 is connected to the filament of bulb 140 by means ofamplifier 142 and integrator 144. g

A blood flow signal path is provided from electrodes 26-to averager 132through amplifier 146 and the variable re-' sistance of cell 134.

In operation, when there is a change in position by the paduced signalpicked up by electrodes 26 is weak, the gain of the circuit will beincreased and when thesignal is strong the gain will be decreased thusmaintaining a signal of constantstrength to averager 132.

Since generally it is only necessary to monitor the blood flow in anygiven artery every 20 to 30 minutes and since the measurement intervalby the flowmeter described hereinabove may only be five or ten minutesand can be reduced if the magnetic field is increased, it is possible touse the same averager and indicator system for sequentially monitoringthe blood flow in several vessels. in such a case, a pair of electrodesmust be provided for each artery to be studied, and means must beprovided to switch the input of amplifier 44 from one pair of electrodesto the next one sequentially according to a cycle program.

Although the electromagnetic flowmeter according to the presentinvention has been particularly described in connection with an analogaverager an analog to digital converter in combination with a digitalcomputer properly synchronized by the signal of the referenceelectrocardiogram may be sub stituted for the analog averager.

What is claimed is:

1. An electromagnetic flowmeter for measuring the blood flow pulse in ablood vessel of a living being comprising first sensing means adapted tobe positioned on the being at a location Where a strong and sharpcardiogram pulse can be repeatedly obtained, means connected to saidfirst sensing means for providing a synchronizing signal and a clock,means for producing a strong and homogeneous magnetic field in theregion of said vessel, means for reversing said magnetic field after agiven number of heart cycles in accordance with a program controlled bysaid synchronizing signal, second sensing means adapted to be placed onsaid being at a location adjacent said vessel, amplifier means foramplifying the signal sensed by said second sensing means, means toparalyze said amplifier means and to reverse its output during reversalof said magnetic field and means synchronized by the synchronizingsignal for averaging the pulsatile signal sensed by said second sensingmeans during successive heart cycles so as to extract the repetitivewave shape of the blood flow pulses from the random noise in which it isotherwise buried.

2. The electromagnetic flowmeter of claim 1 wherein said first sensingmeans comprises a pair of electrodes.

3. The electromagnetic flowmeter of claim 1 wherein said magnetic fieldproducing means comprises an electromagnet having at least one coil oflarge enough size having enough turns of wire to produce said strong andhomogeneous magnetic field.

4. The electromagnetic flowmeter of claim 3 including a source of DCcurrent to feed DC current to said at least one coil.

5. The electromagnetic flowmeter of claim 4 wherein said magnetic fieldreversing means includes means to reverse the polarity of the DC currentfed to said at least one coil repeatedly after a given number of heartcycles in accord with said program controlled by said synchronizingsignal.

6. The electromagnetic flowmeter of claim 5 wherein said paralyzingmeans paralyzes said amplification means and reverses its output duringreversal of the DC current fed to said at least one coil.

7. The electromagnetic flowmeter of claim 1 including filtering meansfor eliminating the DC unbalance of said second sensing means and foreliminating high frequency noise from the signal sensed thereby.

8. The electromagnetic flowmeter of claim 1 wherein said second sensingmeans comprises a second pair of electrodes adapted to be placed on thebeing adjacent the blood vessel under study.

9. The electromagnetic flowmeter of claim 1 wherein said averaging meanscomprises analog averaging means.

10. The electromagnetic flowmeter of claim 1 wherein said averagingmeans comprises digital averaging means.

11. The electromagnetic flowmeter of claim 1 including ground electrodemeans adapted to be positioned on said being remote from said first andsecond sensing means and connected to said amplifier means. I

12. The electromagnetic flowmeter of claim 1 including means formaintaining the signal supplied to said averaging means substantiallyconstant despite moderate changes of position by the being in themagnetic field.

13. An electromagnetic flowmeter for measuring the blood flow pulse in ablood vessel of a living being comprising a first pair of electrodesadapted to be positioned on the being at a location where a strong andsharp cardiogram pulse can be repeatedly obtained, means connected tosaid first pair of electrodes for providing a synchronizing signal and aclock, and electromagnet having at least one coil of large enough sizeand having enough turns of wire to be adequate for the production of astrong and homogeneous magnetic field in the region of the blood vesselunder study, a source of DC current to feed DC current to said at leastone coil of said electromagnet, means to reverse the polarity of said DCcurrent after a given number of heart cycles in accordance with aprogram controlled by said synchronizing signal, a second pair ofelectrodes adapted to be placed on said being adjacent said bloodvessel, an amplifier to amplify the signal appearing between said secondpair of electrodes, filtering means for eliminating the DC unbalance ofsaid second pair of electrodes and for eliminating the high frequencynoise from the signal sensed thereby, means to paralyze the amplifierand to reverse its output during current reversal, and meanssynchronized by the synchronizing signal for averaging the pulsatilesignal measured between said second pair of electrodes during successiveheart cycles so as to extract the repetitive wave shape of the bloodflow pulses from the random noise in which it is otherwise buried.

14. The electromagnetic flowmeter of claim 13 wherein said averagingmeans comprises analog averaging means.

15. The electromagnetic flowmeter of claim 13 wherein said averagingmeans comprises digital averaging means.

s i a

1. An electromagnetic flowmeter for measuring the blood flow pulse in ablood vessel of a living being comprising first sensing means adapted tobe positioned on the being at a location where a strong and sharpcardiogram pulse can be repeatedly obtained, means connected to saidfirst sensing means for providing a synchronizing signal and a clock,means for producing a strong and homogeneous magnetic field in theregion of said vessel, means for reversing said magnetic field after agiven number of heart cycles in accordance with a program controlled bysaid synchronizing signal, second sensing means adapted to be placed onsaid being at a location adjacent said vessel, amplifier means foramplifying the signal sensed by said second sensing means, means toparalyze said amplifier means and to reverse its output during reversalof said magnetic field and means synchronized by the synchronizingsignal for averaging the pulsatile signal sensed by said second sensingmeans during successive heart cycles so as to extract the repetitivewave shape of the blood flow pulses from the random noise in which it isotherwise buried.
 2. The electromagnetic flowmeter of claim 1 whereinsaid first sensing means comprises a pair of electrodes.
 3. Theelectromagnetic flowmeter of claim 1 wherein said magnetic fieldproducing means comprises an electromagnet having at least one coil oflarge enough size having enough turns of wire to produce said strong andhomogeneous magnetic field.
 4. The electromagnetic flowmeter of claim 3including a source of DC current to feed DC current to said at least onecoil.
 5. The electromagnetic flowmeter of claim 4 wherein said magneticfield reversing means includes means to reverse the polarity of the DCcurrent fed to said at least one coil repeatedly after a given number ofheart cycles in accord with said program controlled by saidsynchronizing signal.
 6. The electromagnetic flowmeter of claim 5wherein said paralyzing means paralyzes said amplification means andreverses its output during reversal of the DC current fed to said atleast one coil.
 7. The electromagnetic flowmeter of claim 1 includingfiltering means for eliminating the DC unbalance of said second sensingmeans and for eliminating high frequency noise from the signal sensedthereby.
 8. The electromagnetic flowmeter of claim 1 wherein said secondsensing means comprises a second pair of electrodes adapted to be placedon the being adjacent the blood vessel under study.
 9. Theelectromagnetic flowmeter of claim 1 wherein said averaging meanscomprises analog averaging means.
 10. The electromagnetic flowmeter ofclaim 1 wherein said averaging means comprises digital averaging means.11. The electromagnetic flowmeter of claim 1 including ground electrodemeans adapted to be positioned on said being remote from said first andsecond sensing means and connected to said amplifier means.
 12. Theelectromagnetic flowmeter of claim 1 including means for maintaining thesignal supplied to said averaging means substantially constant despitemoderate changes of position by the being in the magnetic field.
 13. Anelectromagnetic flowmeter for measuring the blood flow pulse in a bloodvessel of a living being comprising a first pair of electrodes adaptedto be positioned on the being at a location where a strong and sharpcardiogram pulse can be repeatedly obtained, means connected to saidfirst pair of electrodes for providing a synchronizing signal and aclock, and electromagnet having at least one coil of large enough sizeand having enough turns of wire to be adequate for the production of astrong and homogeneous magnetic field in the region of the blood vesselunder study, a source of DC current to feed DC current to said at leastone coil of said electromagnet, means to reverse the polarity of said DCcurrent after a given number of heart cycles in accordance with aprogRam controlled by said synchronizing signal, a second pair ofelectrodes adapted to be placed on said being adjacent said bloodvessel, an amplifier to amplify the signal appearing between said secondpair of electrodes, filtering means for eliminating the DC unbalance ofsaid second pair of electrodes and for eliminating the high frequencynoise from the signal sensed thereby, means to paralyze the amplifierand to reverse its output during current reversal, and meanssynchronized by the synchronizing signal for averaging the pulsatilesignal measured between said second pair of electrodes during successiveheart cycles so as to extract the repetitive wave shape of the bloodflow pulses from the random noise in which it is otherwise buried. 14.The electromagnetic flowmeter of claim 13 wherein said averaging meanscomprises analog averaging means.
 15. The electromagnetic flowmeter ofclaim 13 wherein said averaging means comprises digital averaging means.